In recent years, from the standpoint of global environmental protection, emission regulations have been enforced worldwide. As a measure therefor, exhaust gas purification catalysts are used in internal-combustion engines. In such exhaust gas purification catalysts, in order to efficiently remove the ternary component of HC (hydrocarbon), CO and NOx in an exhaust gas, noble metals such as Pt, Pd and Rh are used as a catalyst component.
The reason is that exhaust gas purification catalyst in which a metal other than the aforementioned noble metals, for example, a base metal, is supported on a metal oxide carrier commonly used as a carrier in exhaust gas purification catalysts have hitherto exhibited low purification performance for the aforementioned three components in exhaust gases, particularly NOx.
In automobiles using an exhaust gas purification catalyst in which the aforementioned noble metal is supported on a metal oxide carrier, such as gasoline-powered vehicles and diesel-powered vehicles, in order to improve not only catalytic activity but also fuel efficiency, a variety of systems are used. For example, during steady-state operation, combustion is carried out such that the air-fuel ratio is lean (excess oxygen) in order to improve fuel efficiency. However, in order to improve catalytic activity, it is necessary to temporarily carry out catalysis in a stoichiometric (theoretical air-fuel ratio (A/F)=14.6) to rich (excess fuel) condition.
Therefore, in noble metal-based exhaust gas purification catalysts, an exhaust gas purification catalyst in which the effect of the gas composition of the exhaust gas on the catalytic activity is small is demanded.
All of the aforementioned noble metal catalysts have the problem of resource depletion. Therefore, there is a demand for a purification catalyst which utilizes another noble metal such as silver, which is relatively abundant compared the aforementioned Pt, Pd and Rh, individually or in combination with other base metal(s) and which can exhibit purification performance comparable to conventional noble metal-based catalysts.
Accordingly, various improvements have been attempted to exhaust gas purification catalysts comprising two kinds of metals including noble metals.
For example, JP H11-342339A discloses an exhaust gas purification catalyst comprising a first catalyst for oxidizing NO to NO2 and a second catalyst which cracks HC having a large number of carbon atoms to HC having a small number of carbon atoms and reduces NO2 by allowing the thus cracked HC to react with NO2, the first of these catalyst contains Ag and Ni as catalytic metals, the first catalyst being supported on a carrier and the amount of the supported Ag and Ni being 20 to 60 g and 20 to 50 g, respectively, with respect to 1 L of the carrier. In addition, as a concrete example, it has been shown that Ag—Ni/Al2O3 (co-impregnation) utilizes HC having a large number of carbon atoms and exhibits high NO conversion efficiency (NO-to-NO2 conversion efficiency). However, the aforementioned exhaust gas purification catalyst cannot attain NO2 purification performance in one step.
Further, WO 2008-088027 discloses an exhaust gas purification device which comprises an oxidation catalyst and oxidizes particulate matters (PM) present in exhaust gas using metal oxide particulates, in which oxidation catalyst metal oxide particulates having an average primary particle diameter of 1 to 400 nm are dispersed on a catalytic metal support made of a first metal of at least one selected from the group consisting of Ag, Pt, Rh, Pd, Ru, Ir, Os, Au and Cu, the metal oxide particulates being selected from the group consisting of: oxides in which the metal has a variable valance and is one of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Zr, Fe, Ti, Al, Mg, Co, Ni, Mn, Cr, Mo, W, V, Zn and Sn; solid solutions thereof; and composite oxides thereof. In addition, as a concrete example, it is shown that the exhaust gas purification device, which was obtained by adding an aqueous solution of Ce salt, La salt and Ag salt to ammonia water and coating aggregates prepared from the thus obtained solution by calcination onto DPF (made of cordierite), exhibits favorable PM oxidation performance.
Furthermore, JP 2010-88957A discloses an exhaust gas purification catalyst which comprises a porous metal having pores of 1 to 15 nm, the porous metal containing at least one metal selected from the group consisting of Ru, Os, Rh, Ir, Pd, Pt, Ag and Au and at least one metal selected from the group consisting of Fe, Cu, Co and Ni. In addition, as a concrete example, it is shown that the exhaust gas purification catalyst, which was prepared by annealing and powdering an intermetallic compound obtained by melting and cooling Au and Al in an arc melting furnace device and by subsequently eluting Al with an alkali or acid, has superior CO oxidation performance.